Step-by-step explanation:
Total number of hens John has  = 51 hens
If there are 3 more hens, then total number of hens  = 51 + 3  = 54 hens
Now, one third of the hens are speckled.
[tex](\frac{1}{3}) \times 54 = 18[/tex] Â
⇒ 18 hens are speckled.
Now, [tex](\frac{1}{2})[/tex] of the  total 18 speckled hens  lay speckled eggs.
⇒ 9 hens produce speckled eggs.
Now, one hen lays one egg in a day and a half.
⇒  9  hens lay nine eggs in a day and a half .
⇒  9 hens lay  180 (15 doz.) eggs in [tex]\frac{3}{2} \times\frac{180}{9} = 30[/tex] days
Hence, Â after full 30 days John will have 15 dozen speckled eggs to sell.